This Safari extension is no longer under active development, and will not receive any further updates.
Apple has deprecated the JavaScript API that it relies on, so it will stop working at some point.
Unfortunately, there is no way to get it working again, other than by recreating it from scratch using the new API.

Re-Search

For Safari (OS X)

Repeat your search using a different search engine.

Do you find yourself frequently switching back and forth between search engines? Can’t find the results you’re looking for? Perhaps your default search engine only provides some of the features you want. Now you can easily switch back and forth between results from different search engines with only a single click; no tedious retyping necessary. Re-Search provides a button on the Safari toolbar that will repeat your current web search using the alternative search engine of your choice.

Re-Search lets you choose a search engine to retry with from these familiar options: